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Post by Deleted on Oct 16, 2020 11:13:59 GMT -5
After the amp warms up it loses volume in channel 2. It may play for an hour before it happens then goes in and out. There is no problem on channel 1. Any suggestions about what the problem could be? Any recommendations for an amp tech near Memphis?
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Post by LTB on Oct 16, 2020 16:33:19 GMT -5
After the amp warms up it loses volume in channel 2. It may play for an hour before it happens then goes in and out. There is no problem on channel 1. Any suggestions about what the problem could be? Any recommendations for an amp tech near Memphis? First off, this should be posted in the Amplifier Workshop (Troubleshooting and repair) section. When it goes out, while playing, slightly wiggle each tube one at a time and see if it comes back. If so I suspect the tube socket of that tube needs re-tensioning. ***be sure and use a cloth so you don't burn yourself
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pdf64
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Post by pdf64 on Oct 17, 2020 3:21:47 GMT -5
Have you got any spare 12AX7 that you know are good?
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Post by Deleted on Oct 17, 2020 12:11:57 GMT -5
Sorry about the mispost. I appreciate your input guys. Now I can't make the amp go out. I wiggled all the tubes around and had some crackling in V4 so I replaced that with another 7025 and that stopped. I have a tube caddy full of old tubes for guitar amps but no tester. I'll try again later to make it fail.
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pdf64
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Post by pdf64 on Oct 17, 2020 13:07:25 GMT -5
A guitar amp can make a pretty useful tube tester. Use the V1 socket to check the unknowns. Get your ears tuned into the normal channels response at your preferred control settings. Then flip into standby and fit the unknown condition 12AX7 in there. Flip out of standby and 30 seconds later you should be able to assess that tube for gain, hum, hiss and shot noise etc. Mark up the original tube that was in V1 with a sharpie, so that when you’ve got a pile of tubes, you know which one it was. Mark up the unknowns as good / noisy / low gain etc / dead as appropriate. Noisy tubes can be used in the V5 trem oscillator position, that just needs reasonable gain.
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Post by LTB on Oct 17, 2020 16:41:54 GMT -5
Sorry about the mispost. I appreciate your input guys. No worries, just that you will get more repair related replies by putting it here. As you probably already see.
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Post by Deleted on Oct 18, 2020 15:54:51 GMT -5
Update: Today the amp went out completely on channel 2. I could barely hear it with volume on 10. Channel 1 was still perfect. While this was going on moving the tubes about had no effect on the amp. I switched the amp to standby and back on and channel 2 was back again. Does that suggest anything to anyone? Thanks
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pdf64
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Post by pdf64 on Oct 19, 2020 12:39:46 GMT -5
It may be a 100k plate resistor going intermittently bad? Whatever, it's more than just a bad tube, so time for a tech I'm afraid
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Post by Deleted on Oct 19, 2020 14:20:36 GMT -5
It may be a 100k plate resistor going intermittently bad? Whatever, it's more than just a bad tube, so time for a tech I'm afraid I appreciate your thoughts and agree it's tech time for me. I know some Memphis music shops have techs they have in shop or recommend but I hated to take it to someone I didn't know anything about. I thought about taking the amp out of the case, and turning it on in the dark to see if I see where the fireworks come from. Probably a bad idea. Thanks again.
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Post by Leftee on Oct 19, 2020 15:07:44 GMT -5
There are lethal voltages/current under the chassis. For the untrained, it could be the last idea you have. 😉
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Post by Deleted on Oct 19, 2020 17:41:31 GMT -5
Thanks for the advice Leftee.
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pdf64
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Post by pdf64 on Oct 19, 2020 18:15:46 GMT -5
If you found the problem, what could you do about it, eg can you solder?
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Post by Deleted on Oct 20, 2020 12:47:00 GMT -5
If you found the problem, what could you do about it, eg can you solder? My soldering may not look the best but it's usually functional. I'm no amp tech but have owned tube amps for 57 years. I know the basic stuff. Tubes get hot, don't get killed. I replaced the power cords on three old Fender amps I have. I've done more work on guitars than amps. I may have to open up the hood on this thing and look around. Wish me luck.
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pdf64
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Post by pdf64 on Oct 20, 2020 13:13:33 GMT -5
OK, if you're safe to check some voltages inside an operational amp chassis, measure the Vdc at each of the six 100k plate resistors that form 3 upside down 'V's on the board,marked in red here postimg.cc/PvL51kqHie the Vdc at the eyelet (black probe to the chassis metalwork) where the wires go the terminals 1 and 6 on the V1, V2, V4 tube sockets. Try getting results for when the amp's working normally, and for when the vib channel is wonky. So 12 readings. The 'V' for V1 is adjacent to it, the 'V' for V2 is between V2 and V3, and for V4 it's between V3 and V4.
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Post by Deleted on Oct 22, 2020 9:29:46 GMT -5
OK, if you're safe to check some voltages inside an operational amp chassis, measure the Vdc at each of the six 100k plate resistors that form 3 upside down 'V's on the board,marked in red here postimg.cc/PvL51kqHie the Vdc at the eyelet (black probe to the chassis metalwork) where the wires go the terminals 1 and 6 on the V1, V2, V4 tube sockets. Try getting results for when the amp's working normally, and for when the vib channel is wonky. So 12 readings. The 'V' for V1 is adjacent to it, the 'V' for V2 is between V2 and V3, and for V4 it's between V3 and V4. Sorry I haven't replied. Yesterday was colonoscopy day and the day before was prep. Strangely enough I think I understand most of what you're saying. I'm no electrical engineer but I do have a multi-meter. Am I looking for voltage loss or differences? My amp is an AA763 but your AB568 schematic is probably the same. I'll check. I appreciate your info and will see what I can learn. This project will probably, eventually go to some tech somewhere. Thanks again.
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Post by pdf64 on Oct 22, 2020 11:16:46 GMT -5
I hope the colonoscopy found no problems? I’ve had a sigmoidoscopy, which was fine, all it found were some irritated areas in the rectum, to be expected after radiotherapy to my prostate a few months previously. I actually videoed the screen during the procedure!
Yes, all the plate voltages should be fairly close to each other when the amp is working properly. Best to make the checks with both volumes and the reverb control set to ‘1’, full CCW. Other control settings shouldn’t make much or any difference. We’re looking to see if there’s a drastic change in one of the plate voltages when vibrato channel goes wonky. What sometimes happens is that a resistor fractures, cracks across its body cylinder but its leads and board eyelets still hold it kinda together. So it can become intermittently functional, as temperature changes on the board etc cause the fracture to open and close. Sorry, I meant to note that I only used the SR aa568 because I could find non editable pdfs of the ab763.
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Post by Deleted on Oct 22, 2020 13:05:38 GMT -5
Thanks pdf. Everything came out well with the colonoscopy. No pun intended. It seems rear-end problems come with age. I'm 71. The doctors misplaced my prostate in 2009. Butt, back to the amp. Before I start I may build some kind of wooden stand to hold the chassis securely upside down. Will I need to have the speakers and guitar plugged in to determine when the connection goes wonky? What does full CCW mean? I'll probably wait until tomorrow to get started. I really appreciate the benefit of you knowledge. Thanks again for all your help.
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Post by Deleted on Oct 24, 2020 10:02:28 GMT -5
Update 2: In the last post I meant to say hold the chassis "right side up" instead of "upside down". I assumed full CCW means counter clock-wise. Channel 2 is now completely out so I can only check the plate resistors in the wonky state. The multi-meter needle went wide open at all 6 connections. Does that suggest anything? Thanks.
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Post by LTB on Oct 24, 2020 14:04:07 GMT -5
Update 2: In the last post I meant to say hold the chassis "right side up" instead of "upside down". I assumed full CCW means counter clock-wise. Channel 2 is now completely out so I can only check the plate resistors in the wonky state. The multi-meter needle went wide open at all 6 connections. Does that suggest anything? Thanks. Were you on highest scale? If so they are bad and need replacement
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pdf64
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Post by pdf64 on Oct 25, 2020 12:47:14 GMT -5
Have you got the meter range selected to the highest dc voltage option?
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Post by Deleted on Oct 25, 2020 15:27:46 GMT -5
Thanks for your interest guys. Thinking back I checked the V1 resistor on the lowest setting and the meter pegged out. I moved the settings up and checked again and it still pegged out. I went through the rest of the connections all pegged out. I can't imagine all six resistors going bad. I may have had the meter on high after the first connection. I don't know. Do you think the first resistor could be bad and the others were pegged out because the meter was set too high? Is there a correct sequence to connecting the meter probes to the resistors solder joint and chassis? There was always a pop at the connection that went to terminal 6 on the tube sockets. You can see I'm over my head here. I've put the amp back together and guess I'll wait until I can find a tech in my area to have a look. I don't have much need for amps now anyway and only use the Super for piano and channel 1 still works. I did like a little reverb though. Appreciate your help guys.
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Post by LTB on Oct 25, 2020 18:29:16 GMT -5
Setting meter too low causes it to peg if resistance is higher than that scale if your scale is highest setting and resistance will show closest to left side of meter scale and if you click to next lower scale it will move towards the right and be more accurate. The lower the scale you can get a non maxed reading will be your most accurate. Always start on highest scale and if reading is moves to left click to lowest scale you can get a reading from. Resistors are non polarity devices so doesn’t matter which probe you place on either side. Also a digital volt ohmmeter that is auto ranging would be easier to use but not necessary . What is your highest scale setting ie x1000
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Post by LTB on Oct 26, 2020 0:41:07 GMT -5
If your analog ( needle pointer) meter will not go high enough to read a particular resistor it will show the needle pegged out. I suggest you get a digital volt ohm multi meter ( DVM) that will read in the megohm ( million ohms ) resistance .
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pdf64
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Post by pdf64 on Oct 26, 2020 5:28:03 GMT -5
We’re looking for dc voltage NOT resistance here. Setting an analog meter to a resistance range and trying to measure a dc voltage, especially one up around 200Vdc, could wreck the meter.
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Post by Deleted on Oct 26, 2020 11:25:24 GMT -5
Yeah, my meter is analog and it was set to dc voltage. Is there any scenario where something other than the resistors could be at fault to cause the high readings? As for the meter probes and whether it mattered which connects first I was thinking about a car battery where you would disconnect the ground before the positive to prevent sparks. Would the order have prevented the popping sound I heard when holding a continuous ground and then applying the positive probe? I guess without the speakers I wouldn't have heard it anyway but I had the speakers plugged up and plugged the piano in and turned the volume up before and after the tests to insure it was still wonky. Would it help to pull the amp again and check it more carefully? Thanks again for the info.
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Post by LTB on Oct 26, 2020 13:21:21 GMT -5
ah, my meter is analog and it was set to dc voltage. Is there any scenario where something other than the resistors could be at fault to cause the high readings? So were you reading voltage with the amp powered up, or trying to read ohms with it on DC volts. I am a bit confused
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Post by Deleted on Oct 26, 2020 14:30:13 GMT -5
I guess I'm confused too. Yes, the amp was powered up with the meter on dc voltage but I couldn't read an accurate voltage because the meter was pegged out. Looking at the meter a moment ago it was still set on the lowest dc setting. That may have been the case on the last five resistors but not the first.
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Post by LTB on Oct 27, 2020 0:04:59 GMT -5
If you can get an inexpensive Digital Multimeter. Should suffice for what you are doing and will be easier. Unlike an Analog meter it is not polarity sensitive. If you hook probes backward it will read -volts if you are actually reading a positive voltage and visa versa. They both read up to 600 volts DC Here are two on Amazon. Either should suffice for what you are doing. Or go to Home Depot, Lowes or Harbor Freight Digital MultimeterThis one is Auto Ranging, has a backlit display for easier viewing and a data hold button so while reading voltage, resistance or whatever press button and it locks the reading onto the screen. Being Auto Ranging you don't have to worry about setting the voltage scale. It automatically does it for you. These are only suggestions for you and both have lots of good reviews. Also comes with alligator clip leads that lets you free up your hands and monitor a voltage for a while or whatever Auto Ranging Digital Multimeter
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Post by LTB on Oct 27, 2020 2:27:17 GMT -5
We’re looking for dc voltage NOT resistance here. Setting an analog meter to a resistance range and trying to measure a dc voltage, especially one up around 200Vdc, could wreck the meter. Thanks, this week has been "Real" I jumped to his post about checking wonky resistors without seeing your post telling him to check the DC Voltages on the Plate resisters. Solly for waste of bandwidth here LOL
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pdf64
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Post by pdf64 on Oct 27, 2020 5:17:37 GMT -5
Haha, yes, no prob, pleased you’re back! Thanks for the DMM links, analog meters are too fragile and fininicky.
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